ABSTRACT: Agrobacterium fabrum, the causative agent of crown gall disease, infects a wide range of plants, resulting in significant damage and economical losses worldwide. The bacterium resides in the rhizosphere, where it is exposed to wide variety of biotic and abiotic stressors, including soil contaminants, changes in pH and temperature, and compounds secreted from other bacteria, fungi, or plants. A. fabrum encodes the transcription factor PecS, which has been previously shown to confer fitness in the rhizosphere during the transition from free living stage to the host. The gene encoding pecS is divergent to pecM, which encodes an efflux pump, and the expression of the pecS-pecM gene pair is controlled by PecS. The substrate for PecM is the plant exudate 4-hydroxybenzaldehyde (4HBA), as for instance evidenced by accumulation of 4HBA in a pecM disruption strain. As an aromatic aldehyde, 4HBA is a chemically reactive electrophile that can react with cellular nucleophiles such as thiol groups on proteins and glutathione, leading to disruption of redox homeostasis. To assess consequences of pecM disruption, we compared transcriptome and proteome profiles of WT and pecM disruption strains. While 55 genes were seen to be differentially expressed in the pecM disruption strain based on a log2-fold change of |1|, 118 proteins differentially accumulated based on a log2-fold change of |0.4|. Highly accumulating proteins included glutathione S-transferase and glutathione-dependent peroxiredoxin, both of which protect from oxidative stress. Consistent with this observation, the pecM disrupted strain was seen to be more resistant to oxidative stress. Also notable was a marked depletion of bS21C; this protein is a component of the small ribosomal subunit, where it has been reported to impact translation initiation and fidelity. As one of three homologs, its differential expression speaks to the possibility of a ribosome heterogeneity that is part of adaptation to stress. Taken together, our data suggest that absence of PecM imposes a 4HBA-induced oxidative or electrophile stress, and that PecM serves as a critical metabolic relief valve to maintain cellular homeostasis.
Ahmed Al-Tohamy